1. Field of the Invention
This invention relates to an improvement in doctor blades and the use thereof in doctoring rotating cylinders, moving belts, plates and the like.
2. Description of the Prior Art
Doctoring is a well known procedure which is widely employed in a variety of industrial applications, including web processing, coating and printing, food processing, chemical processing, etc. In a typical doctoring operation, as shown for example in FIGS. 1 and 2, a blade holder 10 has cooperating jaws 12, 14 which define a slot 16 open at one end and closed at the opposite end by a base or seating surface 18. A doctor blade 20 is received in the slot 16. The blade has a front edge which projects from the holder and which is adapted to be applied against the surface to be doctored, which in the case illustrated is the surface 22 of a cylinder rotating in the direction indicated by arrow 24. The doctoring angle "x" may be obtuse, as illustrated, or it may be acute or perpendicular, depending on the overall design of the doctor assembly and the function to be performed by the blade. Typically, obtuse blade angles are used for cleaning and acute blade angles are used for wiping. A perpendicular blade angle is sometimes used for special creping operations.
The doctored surface 22 has a "face width" dimension "w" which is roughly equaled by the length "l" of the doctor blade. In many applications, the doctored surface 22 is essentially straight or flat, in which event the blade holder is normally provided with a straight construction which parallels the doctored surface. In some applications, however, the doctored surface may be crowned, requiring a similar crowning of the holder.
The doctor blades 20 are conventionally manufactured as flat elements, with straight parallel front and rear edges 20a, 20b. During normal operation, the blade is brought into contact with the surface 22 to be doctored and as shown in FIG. 3, an additional loading L.sub.A is applied, as required, to satisfactorily accomplish the intended function, which may for example be surface cleaning, web deflection and handling, creping, metering of inks or web coatings, etc.
As a result of the loading L.sub.A, the doctored surface 22 exerts an equal but opposite force L.sub.O on the blade 20. Moreover, the movement of the surface being doctored generates a second force M which acts essentially perpendicular to force L.sub.O. Force M is the sum of the useful work accomplished by the doctor blade and the frictional resistance to the relative movement between the doctored surface 22 and the front blade edge. The resultant of forces L and M is a force R which is oriented with respect to force M at an angle r.
As shown in FIG. 4, resultant force R may be resolved further into an axial force component P acting in the plane of the blade, and a transverse force component N acting normal to the blade. The sign (.+-.) of transverse force N will change at angle (r+b)=180.degree., thereby signifying that any non-symmetrical blade holder should be inverted to increase proper operation.
The sign of axial force P undergoes a change at angle (b+r)=90.degree.. Within the range of (b+r)&lt;90.degree., force P acts to withdraw the blade from the bladeholder. In such situations, the blade must be restrained by clamping jaws, pins or the like.
Within the range of (b+r)&gt;90.degree. (the condition illustrated in FIG. 4), force P acts to urge the blade into the holder and against the seating surface 18 at the base of the slot 16.
When a doctor blade 20 is loaded against the surface 22 being doctored, the blade acts as a friction brake which consumes power, and most of this power is converted into frictional heat. A portion of the friction heat warms the surface 22, another portion of the friction heat is dissipated into the surrounding air, the processing liquids if present, or the material being doctored, and still another portion of the friction heat warms the contacting surface at the front edge 20a of the doctor blade. The contacting surface of the blade thus becomes a heat source, and this heat is conducted rearwardly towards the rear blade edge 20b. However, because of radiation heat losses occurring at the exposed blade surface and additional heat losses due to conduction through the supporting jaws 12, 14 of the blade holder 10, the rear edge 20b of the blade is usually kept at a lower temperature than the front edge 20a. Thus, during a doctoring operation, there exists a temperature differential between the front and rear edges of the doctor blade.
As the temperature of the blade increases, the blade material will have a tendency to undergo linear expansion. However, because of the temperature differential between the front and rear edges, the front edge will expand more than the rear edge. If the blade were totally unrestrained, it would have a tendency to bow or arc outwardly at the midpoint of its length. However, as explained previously, when the blade is operating in the angular range of (b+r)&gt;90.degree., axial force P urges the blade into the holder and against the seating surface 18, thereby supressing the tendency of the blade to bow or arc outwardly. The warmer front edge portion of the blade is thus not allowed to expand freely, but instead is forced into longitudinal compression. This is an unstable condition which leads to random buckling of the blade front edge. This condition is conventionally referred to as "blade edge heat ripple".
Pronounced blade edge heat ripple can substantially disrupt blade fit and/or uniformity of blade load distribution, which in turn can seriously disrupt the doctoring process. Conventionally, blade edge heat ripple is controlled by increasing load N (see FIG. 4). However, any increase in load N is unavoidably accompanied by a host of serious drawbacks, including increased generation of frictional heat, accelerated wear of the blade and doctored surface, and increased power consumption.